The term “timing” is used in conventional manner to designate all of the opening and closing sequences of the valves of the engine.
It is known both that the performance of a reciprocating engine is highly dependent on the angular positions of the crank shaft at which the valves open and close during the cycle of the engine, and also that the optimum timing diagram changes with changing operating conditions, in particular with changing speed and changing load. Timing has an influence on the actual compression and expansion ratios (Miller cycles), on the quantity of exhaust gas that is optionally recycled, on the energy available from the exhaust gas, on the delivery ratio, on mechanical friction, etc. It is therefore desirable, while the engine is in operation, to be able to modify independently the angular positions of the crank shafts at which the intake and exhaust valves open and close.
In addition, the lift of intake valves strongly influences the turbulence in the combustion chamber, and it may be advantageous to adjust the power of a controlled ignition engine by throttling the admission stream via the intake valve seats rather than by means of a butterfly valve arranged upstream from an admission manifold.
Finally, it may be advantageous to deactivate certain cylinders of an engine by keeping their intake valves and/or their exhaust valves closed during certain stages of operation of the engine.
When the valves are actuated by cam shafts, said valves are pressed by return springs via alternating or sliding mechanisms against rotary cams of profiles that determine the motions in reciprocating translation of said valves. In order to avoid impacts, contact must never be lost between the elements in the drive train that connects together the cams and the valves, and also the valves must land on their seats at a speed that is practically zero.
Most variable devices that have been used in the past generate movement of a valve by using the profile of a single cam that is deformed mechanically or hydraulically.
The complexity of prior art devices for mechanically varying the duration and the lift of valve events (such as the “Valvetronic” device that constitutes the subject matter in particular of patent EP 1 039 103 in the name of BMW) requires very accurate machining, thereby making such devices relatively expensive.
Electrohydraulic devices (such as the “Uniair” device described in particular in patents EP 0 803 642 and EP 1 344 900 in the name of C.R.F.) present the drawback of losing all or some of the energy accumulated in the return springs and of depending largely on the viscosity of the lubricating oil.
Other devices generate valve movement from the profiles of two cams that can be phase-shifted relative to each other, the cams generally being carried by two coaxial shafts, one of which serves to open the valve and the other to close it.
Patent application WO 02/48510 in the name of the Applicant teaches hydraulically mixing the profiles of two phase-shiftable cams in order to modulate the duration of opening, with the drawback of presenting a minimum threshold for valve lift that makes it impossible to reduce the permeability of the orifice controlled by the valve in progressive manner down to total closure thereof.
The present invention relies on the same principle and seeks to provide the same functions as the best prior art devices, by implementing a single rotary phase shifter between two synchronous cam shafts.
The main object of the present invention is thus to be able to act, while in operation, to adjust the angular duration of the opening of at least one valve and also the lift of the valve all the way down to total closure, without losing the energy accumulated in the return means of the device as a result of a hydraulic fluid being throttled.